I have built this circuit but the LED lights even when I move it near DC not only AC.
Does it detect the frequency or the magnetic field?
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3\$\begingroup\$ Magnetic fields that don't change don't affect electric currents. \$\endgroup\$– Criticizing Israel not allowedCommented Aug 27, 2020 at 17:23
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1\$\begingroup\$ if it detects the frequency why does the led lights when move it on DC? even the pen that they sell lights when u move it on DC, the 3xbc547 transistors work here as an ampilifier but i don't know why does it light on DC too not only AC as the diagram says \$\endgroup\$– Patricia SmithCommented Aug 27, 2020 at 17:24
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\$\begingroup\$ If you hold it still near a DC source what happens? \$\endgroup\$– copper.hatCommented Aug 27, 2020 at 17:27
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2\$\begingroup\$ Makes me ask how clean that "DC" source is. I'll bet it isn't pure DC but has some AC noise on it. Try your probe near a battery in the middle of a field well away from power lines... \$\endgroup\$– user16324Commented Aug 27, 2020 at 17:32
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2\$\begingroup\$ Put it near another 9V battery. Hold it still. \$\endgroup\$– copper.hatCommented Aug 27, 2020 at 17:43
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3 Answers
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I have built this circuit but the LED lights even when I move it near DC not only AC
It doesn't surprise me - the copper wire spiral and your hand are integral to make this work - if your hand is picking up AC electrical fields then the DC circuit is acting like a ground to the AC and enough AC voltage will appear across the base-emitter of the left transistor to operate it.
Does it detect the frequency or the magnetic field?
It detects an AC electric field and a higher frequency will be more effective up to a certain point.
answered Aug 27, 2020 at 18:50
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With those three BC547 transistors arranged as they are, that circuit will probably do a pretty good imitation of an electrometer.
A simple electrometer looks like this:
From the above linked web page.
It reacts to electric fields. It can "see" batteries that are just standing there doing nothing.
They can also detect static charges, like an electrostatically charged wool sweater or blanket - or cat.
Being made of bipolar junction transistors (BJT) rather than a junction gate field effect transistor (JFET,) yours probabaly won't be as sensitive as the circuit above made from a JFET.
Your circuit is reacting to the electric fields, not specifically to AC.
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If you bring this detector near a perfect DC source, but your hand is not being held perfectly steady, you are in effect creating an "AC" voltage. As you move closer & further from the DC electric field, the strength of the field changes, which is basically the same effect as if you were near an actual 'AC' source
